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<li class="navelem"><a class="el" href="../../d3/d81/tutorial_contrib_root.html">Tutorials for contrib modules</a></li><li class="navelem"><a class="el" href="../../d3/d00/tutorial_table_of_content_hdf.html">The Hierarchical Data Format (hdf) I/O</a></li>  </ul>
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<div class="title">Creating Groups </div>  </div>
</div><!--header-->
<div class="contents">
<div class="textblock"><h2>Goal </h2>
<p>This tutorial will show you:</p><ul>
<li>How to create a HDF5 file?</li>
<li>How to create a group?</li>
<li>How to check whether a given group exists or not?</li>
<li>How to create a subgroup?</li>
</ul>
<h2>Source Code </h2>
<p>The following code creates two groups: <code>Group1</code> and <code>SubGroup1</code>, where <code>SubGroup1</code> is a child of <code>Group1</code>.</p>
<p>You can download the code from <a href="https://github.com/opencv/opencv_contrib/tree/master/modules/hdf/samples/create_groups.cpp">here</a> or find it in the file <code>modules/hdf/samples/create_groups.cpp</code> of the opencv_contrib source code library.</p>
<div class="fragment"><div class="line"><span class="preprocessor">#include &lt;iostream&gt;</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="../../d0/d9c/core_2include_2opencv2_2core_8hpp.html">opencv2/core.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="../../d7/d55/hdf_8hpp.html">opencv2/hdf.hpp</a>&gt;</span></div><div class="line"></div><div class="line"><span class="keyword">using namespace </span><a class="code" href="../../d2/d75/namespacecv.html">cv</a>;</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main()</div><div class="line">{</div><div class="line"></div><div class="line">    <a class="code" href="../../dc/d84/group__core__basic.html#ga6395ca871a678020c4a31fadf7e8cc63">Ptr&lt;hdf::HDF5&gt;</a> h5io = <a class="code" href="../../d6/dee/group__hdf5.html#ga243d7e303690af3c5c3686ca5785205e">hdf::open</a>(<span class="stringliteral">&quot;mytest.h5&quot;</span>);</div><div class="line"></div><div class="line">    <span class="comment">// &quot;/&quot; means the root group, which is always present</span></div><div class="line">    <span class="keywordflow">if</span> (!h5io-&gt;hlexists(<span class="stringliteral">&quot;/Group1&quot;</span>))</div><div class="line">       h5io-&gt;grcreate(<span class="stringliteral">&quot;/Group1&quot;</span>);</div><div class="line">    <span class="keywordflow">else</span></div><div class="line">       std::cout &lt;&lt; <span class="stringliteral">&quot;/Group1 has already been created, skip it.\n&quot;</span>;</div><div class="line"></div><div class="line">    <span class="comment">// Note that Group1 has been created above, otherwise exception will occur</span></div><div class="line">    <span class="keywordflow">if</span> (!h5io-&gt;hlexists(<span class="stringliteral">&quot;/Group1/SubGroup1&quot;</span>))</div><div class="line">       h5io-&gt;grcreate(<span class="stringliteral">&quot;/Group1/SubGroup1&quot;</span>);</div><div class="line">    <span class="keywordflow">else</span></div><div class="line">       std::cout &lt;&lt; <span class="stringliteral">&quot;/Group1/SubGroup1 has already been created, skip it.\n&quot;</span>;</div><div class="line"></div><div class="line">    h5io-&gt;close();</div><div class="line"></div><div class="line"></div><div class="line">    <span class="keywordflow">return</span> 0;</div><div class="line">}</div></div><!-- fragment --> <h2>Explanation </h2>
<p>First, we create a HDF5 file</p>
<div class="fragment"><div class="line">    Ptr&lt;hdf::HDF5&gt; h5io = <a class="code" href="../../d6/dee/group__hdf5.html#ga243d7e303690af3c5c3686ca5785205e">hdf::open</a>(<span class="stringliteral">&quot;mytest.h5&quot;</span>);</div></div><!-- fragment --><p> If the given file does not exist, it will be created. Otherwise, it is open for read and write.</p>
<p>Next, we create the group <code>Group1</code></p>
<div class="fragment"><div class="line">    <span class="comment">// &quot;/&quot; means the root group, which is always present</span></div><div class="line">    <span class="keywordflow">if</span> (!h5io-&gt;hlexists(<span class="stringliteral">&quot;/Group1&quot;</span>))</div><div class="line">       h5io-&gt;grcreate(<span class="stringliteral">&quot;/Group1&quot;</span>);</div><div class="line">    <span class="keywordflow">else</span></div><div class="line">       std::cout &lt;&lt; <span class="stringliteral">&quot;/Group1 has already been created, skip it.\n&quot;</span>;</div></div><!-- fragment --><p> Note that we have to check whether <code>/Group1</code> exists or not using the function <a class="el" href="../../d3/d23/classcv_1_1hdf_1_1HDF5.html#add5b1b96093a2a28994627cd545e3d9e" title="Check if label exists or not. ">cv::hdf::HDF5::hlexists()</a> before creating it. You can not create a group with an existing name. Otherwise, an error will occur.</p>
<p>Then, we create the subgroup named <code>Subgroup1</code>. In order to indicate that it is a sub group of <code>Group1</code>, we have to use the group name <code>/Group1/SubGroup1</code>:</p>
<div class="fragment"><div class="line">    <span class="comment">// Note that Group1 has been created above, otherwise exception will occur</span></div><div class="line">    <span class="keywordflow">if</span> (!h5io-&gt;hlexists(<span class="stringliteral">&quot;/Group1/SubGroup1&quot;</span>))</div><div class="line">       h5io-&gt;grcreate(<span class="stringliteral">&quot;/Group1/SubGroup1&quot;</span>);</div><div class="line">    <span class="keywordflow">else</span></div><div class="line">       std::cout &lt;&lt; <span class="stringliteral">&quot;/Group1/SubGroup1 has already been created, skip it.\n&quot;</span>;</div></div><!-- fragment --><p> Note that before creating a subgroup, we have to make sure that its parent group exists. Otherwise, an error will occur.</p>
<p>In the end, we have to close the file</p>
<div class="fragment"><div class="line">    h5io-&gt;close();</div></div><!-- fragment --> <h2>Result </h2>
<p>There are many tools that can be used to inspect a given HDF file, such as HDFView and h5dump. If you are using Ubuntu, you can install them with the following commands:</p>
<div class="fragment"><div class="line">sudo apt-get install hdf5-tools hdfview</div></div><!-- fragment --><p>There are also binaries available from the The HDF Group official website <a href="https://support.hdfgroup.org/HDF5/Tutor/tools.html">https://support.hdfgroup.org/HDF5/Tutor/tools.html</a>.</p>
<p>The following figure shows the result visualized with the tool HDFView:</p>
<div class="image">
<img src="../../create_groups.png" alt="create_groups.png"/>
<div class="caption">
Figure 1: Results of creating groups and subgroups</div></div>
<p> The output for <code>h5dump</code> is:</p>
<div class="fragment"><div class="line">$ h5dump mytest.h5</div><div class="line">HDF5 &quot;mytest.h5&quot; {</div><div class="line">GROUP &quot;/&quot; {</div><div class="line">   GROUP &quot;Group1&quot; {</div><div class="line">      GROUP &quot;SubGroup1&quot; {</div><div class="line">      }</div><div class="line">   }</div><div class="line">}</div><div class="line">}</div></div><!-- fragment --> </div></div><!-- contents -->
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